Recent rapid progress in semiconductor device integration demands smaller and smaller wiring patterns or interconnections, and narrower spaces between interconnections which connect active areas. One of the processes available for forming such interconnections is photolithography. Though the photolithographic process can form interconnections that are at most 0.5 microns wide, it requires that surfaces on which pattern images are to be focused by a stepper be as flat as possible because the depth of focus of the optical system is relatively small.
It is therefore necessary to make the surfaces of semiconductor wafers flat for photolithography. One customary way of flattening the surfaces of semiconductor wafers is by Chemical Mechanical Planarization (CMP), which is a process whereby semiconductor wafers are polished with a polishing apparatus.
Conventionally, a CMP polishing apparatus has a turntable and a wafer carrier which rotate at respective individual speeds. A polishing pad is attached to the upper surface of the turntable. A semiconductor wafer seated in the carrier is lowered into engagement with the polishing pad, and clamped between the carrier and the turntable, typically through the exertion of downward force by the carrier. An abrasive grain containing liquid (known as slurry) is deposited onto the polishing pad and retained on the polishing pad. During operation, the carrier exerts a certain pressure on the turntable, and the surface of the semiconductor wafer held against the polishing pad is therefore polished by a combination of chemical polishing and mechanical polishing to a flat mirror finish while the carrier and the turntable are rotated.
The semiconductor wafer that has been polished carries abrasive liquid and ground-off particles attached thereto. Therefore, after polishing, the semiconductor wafer is cleaned and dried in one or more cycles and then housed in a clean storage cassette. If the wafer is not cleaned immediately, the slurry and foreign particles applied to the lower surface of the wafer tend to solidify, becoming very difficult to remove. Also, the known standard cleaning processes, employing, for example, roller brush box type cleaners, are largely ineffective at removing submicron scratches left on the wafer surface by the polishing process.
Thus, additional processing is typically done prior to the wafer cleaning step. For example, a second polish turntable with a second carrier may be employed, using a relatively soft buffing pad in combination with a cleaning chemical, or ultra pure water alone. The buffing process can be effective at removing the residual slurry and buffing out the surface scratches left from the polishing process before cleaning the wafer. However, the effectiveness of the buffing process is also affected by the length of time that slurry sits on the wafer between the polish and buffing process. Unfortunately, adding the buffing process necessitates additional wafer handling and transferring capability, increased tool foot print, and often reduced wafer throughput as a result.
Alternatively, the slurry and surface scratches maybe removed through use of a Hydrofluoric (HF) acid etching process. In such a process, the wafer may be dipped in a bath of the HF acid solution and/or cleaned with an HF solution in a somewhat conventional brush box. However, HF acid poses serious health risks. Compliance with industry safety standards governing the use of HF acid adds substantially to the cost of the equipment and the facility which houses the equipment when employing these techniques.